[PATCH v3 5/5] efi: Use generic EFI loader for x86_64 and i386

[Ard Biesheuvel <ardb@kernel.org>]

Switch the x86 based EFI platform builds to the generic EFI loader,
which exposes the initrd via the LoadFile2 protocol instead of the
x86-specific setup header. This will launch the Linux kernel via its EFI
stub, which performs its own initialization in the EFI boot services
context before calling ExitBootServices() and performing the bare metal
Linux boot.

Given that only Linux kernel versions v5.8 and later support this initrd
loading method, the existing x86 loader is retained as a fallback, which
will also be used for Linux kernels built without the EFI stub. In this
case, GRUB calls ExitBootServices() before entering the Linux kernel,
and all EFI related information is provided to the kernel via struct
boot_params in the setup header, as before.

Note that this means that booting EFI stub kernels older than v5.8 is
not supported even when not using an initrd at all. Also, the EFI
handover protocol, which has no basis in the UEFI specification, is not
implemented.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
 grub-core/Makefile.core.def   |  2 +
 grub-core/loader/efi/linux.c  | 40 +++++++++++++++++++-
 grub-core/loader/i386/linux.c |  8 ++++
 include/grub/efi/efi.h        |  2 +-
 4 files changed, 50 insertions(+), 2 deletions(-)

diff --git a/grub-core/Makefile.core.def b/grub-core/Makefile.core.def
index 1d88c4d1c2555f7f..e58b511242618df7 100644
--- a/grub-core/Makefile.core.def
+++ b/grub-core/Makefile.core.def
@@ -1853,6 +1853,8 @@ module = {
   loongarch64 = loader/efi/linux.c;
   riscv32 = loader/efi/linux.c;
   riscv64 = loader/efi/linux.c;
+  i386_efi = loader/efi/linux.c;
+  x86_64_efi = loader/efi/linux.c;
   emu = loader/emu/linux.c;
   common = loader/linux.c;
   common = lib/cmdline.c;
diff --git a/grub-core/loader/efi/linux.c b/grub-core/loader/efi/linux.c
index 15e0686549d7ecca..bb44de6beec73378 100644
--- a/grub-core/loader/efi/linux.c
+++ b/grub-core/loader/efi/linux.c
@@ -69,6 +69,12 @@ static initrd_media_device_path_t initrd_lf2_device_path = {
   }
 };
 
+extern grub_err_t
+grub_cmd_linux_x86_legacy (grub_command_t cmd, int argc, char *argv[]);
+
+extern grub_err_t
+grub_cmd_initrd_x86_legacy (grub_command_t cmd, int argc, char *argv[]);
+
 static grub_efi_status_t __grub_efi_api
 grub_efi_initrd_load_file2 (grub_efi_load_file2_t *this,
                             grub_efi_device_path_t *device_path,
@@ -125,6 +131,7 @@ grub_arch_efi_linux_load_image_header (grub_file_t file,
   return GRUB_ERR_NONE;
 }
 
+#if !defined(__i386__) && !defined(__x86_64__)
 static grub_err_t
 finalize_params_linux (void)
 {
@@ -169,6 +176,7 @@ failure:
   grub_fdt_unload();
   return grub_error(GRUB_ERR_BAD_OS, "failed to install/update FDT");
 }
+#endif
 
 grub_err_t
 grub_arch_efi_linux_boot_image (grub_addr_t addr, grub_size_t size, char *args)
@@ -231,8 +239,10 @@ grub_arch_efi_linux_boot_image (grub_addr_t addr, grub_size_t size, char *args)
 static grub_err_t
 grub_linux_boot (void)
 {
+#if !defined(__i386__) && !defined(__x86_64__)
   if (finalize_params_linux () != GRUB_ERR_NONE)
     return grub_errno;
+#endif
 
   return (grub_arch_efi_linux_boot_image((grub_addr_t)kernel_addr,
                                           kernel_size, linux_args));
@@ -253,7 +263,9 @@ grub_linux_unload (void)
   if (kernel_addr)
     grub_efi_free_pages ((grub_addr_t) kernel_addr,
 			 GRUB_EFI_BYTES_TO_PAGES (kernel_size));
+#if !defined(__i386__) && !defined(__x86_64__)
   grub_fdt_unload ();
+#endif
 
   if (initrd_lf2_handle != NULL)
     {
@@ -269,6 +281,7 @@ grub_linux_unload (void)
   return GRUB_ERR_NONE;
 }
 
+#if !defined(__i386__) && !defined(__x86_64__)
 /*
  * As per linux/Documentation/arm/Booting
  * ARM initrd needs to be covered by kernel linear mapping,
@@ -304,6 +317,7 @@ allocate_initrd_mem (int initrd_pages)
 				       GRUB_EFI_ALLOCATE_MAX_ADDRESS,
 				       GRUB_EFI_LOADER_DATA);
 }
+#endif
 
 static grub_efi_status_t __grub_efi_api
 grub_efi_initrd_load_file2 (grub_efi_load_file2_t *this,
@@ -345,7 +359,7 @@ static grub_err_t
 grub_cmd_initrd (grub_command_t cmd __attribute__ ((unused)),
 		 int argc, char *argv[])
 {
-  int initrd_size, initrd_pages;
+  int __attribute__ ((unused)) initrd_size, initrd_pages;
   void *initrd_mem = NULL;
   grub_efi_boot_services_t *b = grub_efi_system_table->boot_services;
   grub_efi_status_t status;
@@ -356,6 +370,11 @@ grub_cmd_initrd (grub_command_t cmd __attribute__ ((unused)),
       goto fail;
     }
 
+#if defined(__i386__) || defined(__x86_64__)
+  if (!initrd_use_loadfile2)
+    return grub_cmd_initrd_x86_legacy (cmd, argc, argv);
+#endif
+
   if (!loaded)
     {
       grub_error (GRUB_ERR_BAD_ARGUMENT,
@@ -391,6 +410,7 @@ grub_cmd_initrd (grub_command_t cmd __attribute__ ((unused)),
       return GRUB_ERR_NONE;
     }
 
+#if !defined(__i386__) && !defined(__x86_64__)
   initrd_size = grub_get_initrd_size (&initrd_ctx);
   grub_dprintf ("linux", "Loading initrd\n");
 
@@ -410,6 +430,7 @@ grub_cmd_initrd (grub_command_t cmd __attribute__ ((unused)),
   initrd_end = initrd_start + initrd_size;
   grub_dprintf ("linux", "[addr=%p, size=0x%x]\n",
 		(void *) initrd_start, initrd_size);
+#endif
 
  fail:
   grub_initrd_close (&initrd_ctx);
@@ -442,7 +463,24 @@ grub_cmd_linux (grub_command_t cmd __attribute__ ((unused)),
   kernel_size = grub_file_size (file);
 
   if (grub_arch_efi_linux_load_image_header (file, &lh) != GRUB_ERR_NONE)
+#if !defined(__i386__) && !defined(__x86_64__)
     goto fail;
+#else
+    goto fallback;
+
+  if (!initrd_use_loadfile2)
+    {
+      /*
+       * This is a EFI stub image but it is too old to implement the LoadFile2
+       * based initrd loading scheme, and Linux/x86 does not support the DT
+       * based method either. So fall back to the x86-specific loader that
+       * enters Linux in EFI mode but without going through its EFI stub.
+       */
+fallback:
+      grub_file_close (file);
+      return grub_cmd_linux_x86_legacy (cmd, argc, argv);
+    }
+#endif
 
   grub_loader_unset();
 
diff --git a/grub-core/loader/i386/linux.c b/grub-core/loader/i386/linux.c
index c2385d0a561a8fc4..997647a33326eeb8 100644
--- a/grub-core/loader/i386/linux.c
+++ b/grub-core/loader/i386/linux.c
@@ -1136,6 +1136,7 @@ grub_cmd_initrd (grub_command_t cmd __attribute__ ((unused)),
   return grub_errno;
 }
 
+#ifndef GRUB_MACHINE_EFI
 static grub_command_t cmd_linux, cmd_initrd;
 
 GRUB_MOD_INIT(linux)
@@ -1152,3 +1153,10 @@ GRUB_MOD_FINI(linux)
   grub_unregister_command (cmd_linux);
   grub_unregister_command (cmd_initrd);
 }
+#else
+extern grub_err_t __attribute__((alias("grub_cmd_linux")))
+grub_cmd_linux_x86_legacy (grub_command_t cmd, int argc, char *argv[]);
+
+extern grub_err_t __attribute__((alias("grub_cmd_initrd")))
+grub_cmd_initrd_x86_legacy (grub_command_t cmd, int argc, char *argv[]);
+#endif
diff --git a/include/grub/efi/efi.h b/include/grub/efi/efi.h
index 7c12cb9aa91040e0..d3b96761947a0944 100644
--- a/include/grub/efi/efi.h
+++ b/include/grub/efi/efi.h
@@ -113,12 +113,12 @@ extern void (*EXPORT_VAR(grub_efi_net_config)) (grub_efi_handle_t hnd,
 #if defined(__arm__) || defined(__aarch64__) || defined(__riscv) || defined(__loongarch__)
 void *EXPORT_FUNC(grub_efi_get_firmware_fdt)(void);
 grub_err_t EXPORT_FUNC(grub_efi_get_ram_base)(grub_addr_t *);
+#endif
 #include <grub/file.h>
 grub_err_t grub_arch_efi_linux_load_image_header(grub_file_t file,
                                                 struct linux_arch_kernel_header *lh);
 grub_err_t grub_arch_efi_linux_boot_image(grub_addr_t addr, grub_size_t size,
                                            char *args);
-#endif
 
 grub_addr_t grub_efi_modules_addr (void);
 
-- 
2.39.2